Increasing the dielectric strength in metal electrodes

ABSTRACT

For increasing the dielectric strength in metal electrodes, particularly in parts of the electrodes around which there prevail inhomogeneous electrical fields with uncontrollable electrical field strength, a layer of resin with a dielectric constant which is high in comparison with the dielectric constant of transformer oil is applied to the electrode by means of electrostatic spraying.

United States Patent Andersson et a1.

INCREASING THE DIELECTRIC STRENGTH IN METAL ELECTRODES Inventors: Olle Andersson; Ellerth Ericsson;

Svante Forsberg, all of Ludvika,

Sweden Assignec: Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden Filed:

June 19, 1974 Appl. No.: 480,929

Foreign Application Priority Data June 20, 1973 Sweden 7308658 U.S. Cl 336/84; 117/934 R; 174/127;

174/142 Int. Cl. l-IOIF 27/34; H0113 17/44 Field of Search. 174/73 R, 127, 140 R, 140'C, 174/140 H, 140 CR, 142, 143, 144; 310/196;

336/84, 87, 219; l17/93.4 R,93.4 A

Primary Examiner-Lara'mie E. Askin 5 7 ABSTRACT For increasing the dielectric strength in metal electrodes, particularly in parts of the electrodes around which there prevail inhomogeneous electrical fields with uncontrollable electrical field strength, a layer of resin with a dielectric constant which is high in comparison with the dielectric constant of transformer oil is applied to the electrode by means of electrostatic spraying.

3 Claims, 6 Drawing Figures IF I II l I 3 4 2 1 I 2 l 22 l I I I l I I z I I l 22 i g i 1 1 I U.S. Patsnt Oct. 7,1975 Sheet 1 of2 3,911,384

INCREASING THE DIELECTRIC STRENGTH IN METAL ELECTRODES BACKGROUND OF THE INVENTION which there prevail inhomogeneous electric fields with 1 uncontrollable electric field strength. The term electrodes must here be taken in its broad sense, and in this context it comprises construction parts for electric machines and apparatus which have high voltage to earth or other live parts or earth-connected parts which are located in the vicinity of live parts. As examples of what is meant may be mentioned the iron core with relevant parts, splash rings for live parts and the like.

In the manufacture of, for example, iron cores for transformers, the sheets will display small irregularities in the cut edges. Even such construction parts as press beams, lifting devices and the like cannot be made absolutely smooth, and even after a normal grinding there will be irregularities which, in normal cases, are of no significance. However, if a conductor with high voltage passes in the vicinity of an earthed object displaying such insignificant irregularities on the surface, the electric field between the conductor and the earthed object will be affected very strongly quite close to the irregularities, and the insulating material just outside even an almost microscopic elevation will be exposed to a considerably increased field strength with a resultant risk of glow discharge appearing there. Such a seemingly insignificant partial discharge can easily increase and will eventually give rise to a flash-over in the liquid insulating agent between the earthed object and the live conductor.

Tests have shown that, if the parts between which the electric field occurs are coated with a thin layer of a resin, the dielectric constant of which is higher and preferably considerably higher than the dielectric constant of the liquid insulating agent, the said phenomenon with partial discharges at the metal surfaces will practically disappear. In this connection it is important to ensure that the layer is thin and that it covers the irregularities present on the surfaces only. Further it is important that the surface of the applied resin has no .sharp comers and edges.

SUMMARY OF THE INVENTION According to the present invention, the problem of avoiding partial discharges on the electrode surfaces referred to above has been solved by applying a layer of a resin with a dielectric constant which is high in comparison with the dielectric constant of the transformer oil to the electrode by means of electrostatic spraying.

Electrostatic spray painting as such is well-known, but so far it has not been known to employ this method for strengthening the capacity of the electrodes to endure strong electrical fields in electrical apparatus. The method according to the invention makes use of the tendency of the applied material to accumulate mostly where the electrical field strength is highest, which is where the strongest insulation is required.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings,

FIG. 1 shows a sharp corner 'in a metallic construction part coated with an insulating-agent according to the invention;

FIG. 2 shows a vertical section through a toroidal splash ring on the line II-II of FIG. 3.

FIG. 3 is a view from above of the splash ring accord- 0 ing to FIG. 2.

FIG. 4 shows an enlarged cross-section of the area IV in FIG. 2 and also an enlarged view of the encircled area II in FIG. 6.

FIG. 5 shows a time-temperature curve for the application of a coating according to the invention.

FIG. 6 shows a schematic view of a transformer to which the invention is applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a sharp corner in a metallic construction part 1 in an electrical apparatus. This may be, for example, the iron core in a transformer. When cutting transformer sheet, burrs will always appear and these will have a particular importance at the corners of the construction, with a resultant risk of glow phenomena. To avoid glow discharge and to strengthen the electric strength, an insulating layer 2 is applied by means of electrostatic spraying of a powdered resin or a resinous solution. Because of the increased field strength at the corner 3, an extra thick coating will appear there. This means that a coating is obtained, the thickness of which is related to the risk of flash-over between the construction part and a live conductor in the vicinity thereof.

FIG. 2 shows a vertical section through a toroidal splash ring 4, which is placed on that end of a transformer bushing which is located in the oil in a transformer tank. The lower end of the bushing is indicated at 5. The splash ring is supported by the bushing by means of stays 6. FIG. 3 shows the splash ring from above. Such splash rings are manufactured of a thin material, and the field strength will thus be high along the edges 7 with a subsequent risk of glow discharge. FIG. 4 shows a section through the edge 7 on an enlarged scale, after the splash ring has been treated according to the invention. The figure shows that the said edge is coated with a layer 8 of insulating material so that the otherwise sharp edge has a considerably increased radius of curvature. The risk of glow discharge and flash-over at the edge is thus considerably reduced.

The treatment with resin according to the invention causes all sharp edges to acquire a smooth curvature with a definable radius. This decreases the field strength and reduces the risk of glow discharge and flash-over.

In order that the powder may stick and be evenly distributed, the object to be sprayed is preheated to a temperature exceeding the melting point of the powder. FIG. 5 shows the time temperature curve during the process of coating an object. The straight part 10 indicates the temperature of the object before the start of the spraying. At 11 the object is removed from the heating furnace and the spraying commences. While the spraying is going on, the temperature decreases, which is marked by the downward curve part 12. At 13 the spraying is finished and the applied resin must be cured and this curing can be performed under a continuing temperature drop in the object according to one of the curve parts 14 or 15. It is also possible to place the object in a furnace so that the curing continues at constant temperature, as shown by the curve part 16. In any event it mustbe ensured that the temperature does not rise after the spraying has been finished.

FIG. 6 shows a transformer having a tank filled with oil. An iron core 21 with windings 22 is in the tank. In the top lid of the tank there are three bushings 23. The lower end of each bushing situated in the tank has a splash ring 4, and each bushing is connected to a winding by a conductor 24. The encircled area II at the lower end of the bushing is shown on an enlarged scale in FIG. 2. The encircled area I at the upper righthand corner is shown in FIG, 1 and explained in connection with the description of said Figure,

In the foregoing it has been assumed that the spraying is carried out with resin in the form of powder, but it is also possible to spray with solutions of powdered resin.

insulating liquid, said layer being substantially thicker at such portions exposed to the greater than at such other portions..-

2. Apparatus as claimed in claim 1, in which said layer is produced by electrostatic spraying.

3. An arrangement as claimed in claim 1, in which said apparatus is a transformer.

field strength 

1. In an electrical apparatus, first and second members formed of conductive material with an insulating liquid therebetween, one of said members being a live member, one of the members having a surface exposed to the insulating liquid, said surface having portions which are exposed to a considerably greater field strength than other portions of the surface when a high voltage exists in such live element, and a layer on such surface of a resin with a dielectric constant which is high in comparison with the dielectric constant of the insulating liquid, said layer being substantially thicker at such portions exposed to the greater field strength than at such other portions.
 2. Apparatus as claimed in claim 1, in which said layer is produced by electrostatic spraying.
 3. An arrangement as claimed in claim 1, in which said apparatus is a transformer. 